Recently, in the production of monosodium glutamate, citric acid, and the enzymic preparation of the foodstuff industry, as well as in medical application, both filtrators of relatively large size with filters constituted of active carbon, wood flour, cotton, etc. and filtrators of relatively small size with high efficiency .mu.m level have been used. The latter has the problem of the filtering orifices becoming blocked and thus being blocked and subject to a shorter operative life. The small diameter of the membrane orifices, moreover, is manufactured as small as 0.1 .mu.m, and the degree of the purity of the filtered air has been unsatisfactory.